Patrick,
Nice to see a German Yamaha!!
Here in the States, the Yamaha's are getting pretty rare.
The 12.5 volts looks to be the problem with your clutch. I would suspect the rectifier/regulator
assembly, which is a pretty expensive part. First I would make sure your battery is in good shape.
Take it out of the mower, clean all connections, and load-test it to make sure it's ok, or just get
a new one. Make sure the ground wire is making good contact with the chassis. The battery when
tractor not running should be ~12v, but once you start it, voltage should go to ~14v indicating proper
charging. Dirty connections can cause this too, so just make sure all battery connections are good.
Be interesting to see the new color, as you don't see any with a different color scheme.
Rattosh51
YT6800